The invention relates to the production of terephthalates of alkaline or alkaline-earth metals from polyol polyterephthalate and in particular from glycol polyterephthalate (P.E.T.).
The difficulties of recycling P.E.T.-type polyesters are due mainly to the coloring and to the foreign substances that are found mixed, with these products, and to the difficulty of elimination of the water marks in the recycled product.
One of the best forms of recovery is a depolymerization and recovery from terephthalic acid and glycol in the form of raw materials that can result in new polyesters of the same purity as the products produced from new raw materials.
The breakdown of the P.E.T. and of the polyesters having terephthalic acid as a dibasic acid is described in the prior art as being performed in an aqueous soda solution containing 3 to 20% by weight of soda, at a pressure of 1.38 to 2.07 MPa (200 to 300 PSI) and at a temperature between 180.degree. and 320.degree. C., preferably between 210.degree. and 250.degree. C. (U.S. Pat. No. 3,317,519).
In batch installations, this process results in a considerable use of energy, and requires a material that must withstand the pressure.
The hydrolysis periods are long, from 3 to 5 hours, and the amount of soda used is on the order of 1.5 in comparison with the theory.
According to another proposal (GB-A-822,834), it is desired to obtain salting out of the terephthalic acid salt either by operating at 100.degree. C. with a relatively dilute aqueous solution of sodium hydroxide or by operating in the presence of an alcohol. The presence of an alcohol in the present invention is not necessary and is even considered detrimental.